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Income taxes - Additional Information (Detail) (USD $)
6 Months Ended
Jun. 30, 2012
Week
Jun. 30, 2011
Income Taxes [Line Items]    
Effective tax rate 38.00% 35.00%
Number of weeks of new hire employment required for new hire retention credit 52  
Maximum
   
Income Taxes [Line Items]    
Tax credit for each worker under new hire retention credit   1,000